Events doc fixes

Documentation fix for #8853 ... specifically, acknowledge in the docs that EE.addListener(...) does not check to see if the given listener has already been added, and that newListener will be called regardless of whether an already existing listener is added twice.
This is borderline misleading... it seems to me to almost suggest that `.on()` doesn't really add listeners if they are already in the array, something which isn't true (as you emphasized above).